1. Field of the Invention
The present invention relates to an apparatus and a method for transmitting data in a communication system; and, more particularly, to an apparatus and a method for transmitting data in a wireless communication system.
2. Description of Related Art
In general, a communication system is largely classified into a wired communication system and a wireless communication system. According to the wired communication system, since its connection between a terminal and a network is accomplished through a wire, stable and fast data transmission is possible but there is limitation in a user's activity range. On the contrary, according to the wireless communication system, data are transmitted using a predetermined frequency through a connection between a terminal and a network. As a result, the wireless communication system is relatively slow compared to the wired communication system but there is no limitation in the activity range of a user.
The packet-based high speed wireless communication systems are currently available in the wireless communication system due to the rapid development of techniques. Additionally, various techniques have been continuously studied. Furthermore, techniques, which are applied to channel conditions by controlling a transmission power, have been mainly used in order to effectively transmit data in a high speed. However, if these techniques are solely used, there are various limitations in increasing a packet transmission rate. Therefore, methods capable of increasing a packet transmission rate through more various techniques become necessary.
In order to meet these demands, methods capable of increasing a packet transmission rate through various techniques have been studied and introduced. The wireless communication system is mainly divided into a physical layer and a Media Access Control (MAC) layer. In the physical layer, complex techniques are used to improve a data rate and data transmission to a reliable level. A representative technique, which is usually used in the physical layer to constitute an ultra high-speed wireless communication system, includes a high speed wireless transmission method using a multiple antenna technology based on an orthogonal frequency division multiplexing, a channel encoding method, and a high dimensional modulation method. In addition, the MAC layer defines a certain protocol to provide high quality service to a user and mainly uses a Block acknowledgement (Ack) and aggregation mode technique, in order to provide a high throughput. The aggregation mode can transmit one long packet including connected several packets at once.
For example, according to Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11n, which is an international double-balanced standard, a high dimensional modulation method of 64-QAM and a channel coding method having a high code rate and a ⅚code rate are introduced in the physical layer. Moreover, according to IEEE 802.11n, which is an internal wireless Local Area Network (LAN) standard, multiple antennas and a frequency orthogonal modulation method are applied to increase a physical layer data rate up to about 300 Mbps. Furthermore, in order to reduce an overhead caused by an interval between a header and a frame in the MAC layer, the Block Ack method and the aggregation mode are used to adopt a certain technique as a standard. According to the certain technique, a transmission rate of the MAC layer maintains a throughput up to the maximum value of about 200 Mbps if the physical layer rate is about 300 Mbps.
When IEEE, which is standards association of a wireless LAN, employs the Block Ack method and the aggregation mode for a MAC layer standard, a high throughput is theoretically possible. However, this throughput may not be possible in an actual wireless environment. The reason is that a signal is transmitted using the 64_QAM modulation method and ⅚ code rate through a multiple antenna, in order to achieve a throughput of more than about 180 Mbps. Therefore, a channel should be stable during a relatively long packet interval and a signal-to-noise ratio required in a receiver terminal must be maintained during the time interval in the wireless LAN system. Additionally, error propagation caused by tracking error should be relatively small in order to achieve a high throughput in the receiver terminal of the wireless LAN system.
An operation of the aggregation mode will be described below. First, when data to be transmitted are generated in a transmitter, the generated data constitute packets and then, one long packet including the predetermined number of packets is transmitted at one time. Once the data transmission is completed, a receiver demodulates and decodes the received one long packet and detects a reception error. When there is an error in the received one long packet, information about whether there are errors or not in a plurality of packets is notified to the transmitter at one time. When there is a packet having an error in the previously transmitted packets, the transmitter retransmits the same packet, thereby improving a reception success rate of a packet.
However, a retransmission time is one typical factor that deteriorates a throughput when a packet is transmitted. Therefore, in order to transmit data effectively, retransmission needs to be less basically. However, an actual channel has a lot of noise and has a frequency selective characteristic, and a long packet has a high packet error rate (PER) due to a time offset or frequency offset in a Radio Frequency (RF) and analog path. Thus, it is difficult to reach a throughput that can be theoretically obtainable in an ideal environment.